Dynamic I/O load balancing for zHyperLink

ABSTRACT

A method for dynamically balancing I/O workload is disclosed. In one embodiment, such a method includes transmitting, from a host system to a storage system, read requests and write requests over a communication path, such as a zHyperLink communication path. The method further determines whether first and second sets of conditions (e.g., read cache hit ratio, read and write response times, read and write reject rates, etc.) are satisfied on one or more of the host system and storage system. In the event the first set of conditions is satisfied, the method increases a ratio of read requests to write requests that are transmitted over the communication path. In the event the second set of conditions is satisfied, the method decreases the ratio of read requests to write requests that are transmitted over the communication path. A corresponding system and computer program product are also disclosed.

BACKGROUND Field of the Invention

This invention relates to systems and methods for balancing I/O whenusing zHyperLink and other similar I/O processing technologies.

Background of the Invention

When an I/O request is performed by an application, several processesmay be performed to complete the request. These processes affect I/Olatency, which can be a significant part of application response time.zHyperLink is a technology designed to reduce I/O latency by providing afast, reliable, and direct communication path between a host system andstorage system. This is accomplished by installing zHyperLink adapterson the z/OS host system and storage system, and connecting thecomponents together using zHyperLink cables. This configuration createsa point-to-point connection between the host system and storage system,which reduces I/O response times by up to ten times compared to zHigh-Performance FICON® (zHPF). Such low response times are achieved byusing synchronous I/O requests, which reduce the amount of time requiredfor some functions, such as I/O interrupts and z/OS dispatch operations.

Standard I/O processing that is available using technologies, such aszHPF, requires I/O operations to perform a series of time-consumingtasks, such as z/OS dispatching, interrupt handling, CPU queuing, andL1/L2 processor cache reloading. These tasks and others required for I/Oprocessing may cause I/O response times to be relatively long comparedto transferring data within virtual storage, with response times of 130+microseconds plus interrupt handling and CPU dispatch time.

Using zHyperLink, when a synchronous I/O is performed, the CPU on thehost system waits or “spins” until the I/O is complete, or a timeoutvalue is reached. zHyperLink can significantly reduce the time that isrequired to complete the I/O because the dispatching, interrupthandling, CPU queue time, and CPU cache reload activities are no longernecessary. This saves the processor time associated with two contextswaps involved in a normal I/O operation, putting the thread to sleepand then re-dispatching it, as well as performing the I/O interrupt.

In order to achieve the improved I/O response times associated withsynchronous I/O, the code path used to process the I/O needs to behighly optimized. Any conditions that delay a synchronous I/O operation,such as a cache miss, may cause a notification to be returned to a hostsystem and the operation to be retried using a slower communication pathsuch as zHPF. Synchronous I/O is only successful when microcode cancomplete a synchronous I/O operation in a very short amount of time,such as 10-30 microseconds. If the synchronous I/O operation cannot becompleted in that amount of time, it may fail and the host system mayneed to retry the operation over a non-optimal path such as FICON.

Reads and writes performed using synchronous I/O typically compete forthe same set of resources. Under some conditions, it may be advantageousto execute more writes than reads over a zHyperLink path. Under otherconditions, it may be advantageous to execute more reads than writesover the same zHyperLink path. Systems and methods are needed to detectsuch conditions and communicate information about the conditions to ahost system so that the host system can adjust the ratio of reads andwrites accordingly.

SUMMARY

The invention has been developed in response to the present state of theart and, in particular, in response to the problems and needs in the artthat have not yet been fully solved by currently available systems andmethods. Accordingly, systems and methods have been developed todynamically balance I/O workload. The features and advantages of theinvention will become more fully apparent from the following descriptionand appended claims, or may be learned by practice of the invention asset forth hereinafter.

Consistent with the foregoing, a method for dynamically balancing I/Oworkload is disclosed. In one embodiment, such a method includestransmitting, from a host system to a storage system, read requests andwrite requests over a communication path, such as a zHyperLinkcommunication path. The method further determines whether first andsecond sets of conditions (e.g., read cache hit ratio, read and writeresponse times, read and write reject rates, etc.) are satisfied on oneor more of the host system and storage system. In the event the firstset of conditions is satisfied, the method increases a ratio of readrequests to write requests that are transmitted over the communicationpath. In the event the second set of conditions is satisfied, the methoddecreases the ratio of read requests to write requests that aretransmitted over the communication path.

A corresponding system and computer program product are also disclosedand claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered limiting of its scope, the embodiments of the inventionwill be described and explained with additional specificity and detailthrough use of the accompanying drawings, in which:

FIG. 1 is a high-level block diagram showing one example of a networkenvironment in which systems and methods in accordance with theinvention may be implemented;

FIG. 2 is a high-level block diagram showing one example of a storagesystem in the network environment of FIG. 1;

FIG. 3 is a high-level block diagram showing different communicationpaths between a host system and a storage system;

FIG. 4 is a high-level block diagram showing, when a first set ofconditions is satisfied, increasing a ratio of reads to writes over ahigher performance communication path, and decreasing a ratio of readsto writes over a lower performance communication path;

FIG. 5 is a high-level block diagram showing, when a second set ofconditions is satisfied, decreasing a ratio of reads to writes over ahigher performance communication path, and increasing a ratio of readsto writes over a lower performance communication path;

FIG. 6 is a high-level block diagram showing a ratio adjustment modulefor adjusting a ratio of reads and writes over a communication path;

FIG. 7 is a process flow diagram showing one embodiment of a method forincreasing a ratio of reads relative to writes; and

FIG. 8 is a process flow diagram showing one embodiment of a method fordecreasing a ratio of reads relative to writes.

DETAILED DESCRIPTION

It will be readily understood that the components of the presentinvention, as generally described and illustrated in the Figures herein,could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of the invention, as represented in the Figures, is notintended to limit the scope of the invention, as claimed, but is merelyrepresentative of certain examples of presently contemplated embodimentsin accordance with the invention. The presently described embodimentswill be best understood by reference to the drawings, wherein like partsare designated by like numerals throughout.

The present invention may be embodied as a system, method, and/orcomputer program product. The computer program product may include acomputer readable storage medium (or media) having computer readableprogram instructions thereon for causing a processor to carry outaspects of the present invention.

The computer readable storage medium may be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages.

The computer readable program instructions may execute entirely on auser's computer, partly on a user's computer, as a stand-alone softwarepackage, partly on a user's computer and partly on a remote computer, orentirely on a remote computer or server. In the latter scenario, aremote computer may be connected to a user's computer through any typeof network, including a local area network (LAN) or a wide area network(WAN), or the connection may be made to an external computer (forexample, through the Internet using an Internet Service Provider). Insome embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, may be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus, or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

Referring to FIG. 1, one example of a network environment 100 isillustrated. The network environment 100 is presented to show oneexample of an environment where systems and methods in accordance withthe invention may be implemented. The network environment 100 ispresented by way of example and not limitation. Indeed, the systems andmethods disclosed herein may be applicable to a wide variety ofdifferent network environments, in addition to the network environment100 shown.

As shown, the network environment 100 includes one or more computers102, 106 interconnected by a network 104. The network 104 may include,for example, a local-area-network (LAN) 104, a wide-area-network (WAN)104, the Internet 104, an intranet 104, or the like. In certainembodiments, the computers 102, 106 may include both client computers102 and server computers 106 (also referred to herein as “host systems”106). In general, the client computers 102 initiate communicationsessions, whereas the server computers 106 wait for requests from theclient computers 102. In certain embodiments, the computers 102 and/orservers 106 may connect to one or more internal or externaldirect-attached storage systems 110 a (e.g., arrays of hard-disk drives,solid-state drives, tape drives, etc.). These computers 102, 106 anddirect-attached storage systems 110 a may communicate using protocolssuch as ATA, SATA, SCSI, SAS, Fibre Channel, or the like.

The network environment 100 may, in certain embodiments, include astorage network 108 behind the servers 106, such as astorage-area-network (SAN) 108 or a LAN 108 (e.g., when usingnetwork-attached storage). This network 108 may connect the servers 106to one or more storage systems, such as arrays 110 b of hard-disk drivesor solid-state drives, tape libraries 110 c, individual hard-disk drives110 d or solid-state drives 110 d, tape drives 110 e, CD-ROM libraries,or the like. To access a storage system 110, a host system 106 maycommunicate over physical connections from one or more ports on the host106 to one or more ports on the storage system 110. A connection may bethrough a switch, fabric, direct connection, or the like. In certainembodiments, the servers 106 and storage systems 110 may communicateusing a networking standard such as Fibre Channel (FC).

Referring to FIG. 2, one embodiment of a storage system 110 containingan array of hard-disk drives 204 and/or solid-state drives 204 isillustrated. As shown, the storage system 110 includes a storagecontroller 200, one or more switches 202, and one or more storage drives204, such as hard disk drives 204 or solid-state drives 204 (such asflash-memory-based drives 204). The storage controller 200 may enableone or more hosts 106 (e.g., open system and/or mainframe servers 106running operating systems such z/OS, zVM, or the like) to access data inthe one or more storage drives 204.

In selected embodiments, the storage controller 200 includes one or moreservers 206. The storage controller 200 may also include host adapters208 and device adapters 210 to connect the storage controller 200 tohost devices 106 and storage drives 204, respectively. Multiple servers206 a, 206 b may provide redundancy to ensure that data is alwaysavailable to connected hosts 106. Thus, when one server 206 a fails, theother server 206 b may pick up the I/O load of the failed server 206 ato ensure that I/O is able to continue between the hosts 106 and thestorage drives 204. This process may be referred to as a “failover.”

In selected embodiments, each server 206 may include one or moreprocessors 212 and memory 214. The memory 214 may include volatilememory (e.g., RAM) as well as non-volatile memory (e.g., ROM, EPROM,EEPROM, hard disks, flash memory, etc.). The volatile and non-volatilememory may, in certain embodiments, store software modules that run onthe processor(s) 212 and are used to access data in the storage drives204. These software modules may manage all read and write requests tological volumes in the storage drives 204.

One example of a storage system 110 having an architecture similar tothat illustrated in FIG. 2 is the IBM DS8000™ enterprise storage system.The DS8000™ is a high-performance, high-capacity storage controllerproviding disk storage that is designed to support continuousoperations. Nevertheless, the systems and methods disclosed herein arenot limited to operation with the IBM DS8000™ enterprise storage system110, but may operate with any comparable or analogous storage system110, regardless of the manufacturer, product name, or components orcomponent names associated with the system 110. Furthermore, any storagesystem that could benefit from one or more embodiments of the inventionis deemed to fall within the scope of the invention. Thus, the IBMDS8000™ is presented by way of example and is not intended to belimiting.

Referring to FIG. 3, when an I/O request is performed by an applicationresiding on a host system 106, several processes may be performed tocomplete the request. These processes may affect I/O latency andapplication response time. zHyperLink is a technology designed to reduceI/O latency by providing a fast, reliable, and direct communication pathbetween a host system and storage system 110. This may be accomplishedby installing zHyperLink adapters on the host system 106 and storagesystem 110, and connecting the components using zHyperLink cables. Thisconfiguration creates a point-to-point connection 300 between the hostsystem 106 and the storage system controller 200. This may reduce I/Oresponse times by up to ten times compared to using a conventionalcommunication path 302, such as a z High-Performance FICON® (zHPF)communication path 302. Such low response times may be achieved by usingsynchronous I/O requests, which reduce the amount of time required forsome functions, such as I/O interrupts and I/O dispatch operations.

Standard I/O processing that is available using technologies, such aszHPF, requires I/O operations to perform a series of time-consumingtasks, such as z/OS dispatching, interrupt handling, CPU queueing, andL1/L2 processor cache reloading. These tasks and others required for I/Oprocessing may cause I/O response times to be relatively long comparedto transferring data within virtual storage, with response times of 130+microseconds plus interrupt handling and CPU dispatch time.

Using zHyperLink, when a synchronous I/O is performed, the CPU on thehost system 106 may wait or “spin” until the I/O is complete, or atimeout value is reached. zHyperLink can significantly reduce the timethat is required to complete the I/O because the dispatching, interrupthandling, CPU queue time, and CPU cache reload activities are no longernecessary. This reduces processor time needed to perform two contextswaps in a normal I/O operation, putting the thread to sleep and thenre-dispatching it, as well as performing the I/O interrupt.

In order to achieve the improved I/O response times associated withsynchronous I/O, the code path used to process the I/O needs to behighly optimized. Any conditions that delay a synchronous I/O operation,such as a cache miss, may cause a notification to be returned to a hostsystem 106 and the operation to be retried using a slower communicationpath 302 such as zHPF. Synchronous I/O is only successful when microcodecan complete a synchronous I/O operation in a very short amount of time,such as 10-30 microseconds. If a synchronous I/O operation cannot becompleted in that amount of time, the synchronous I/O operation may befailed and the host system 106 may retry the operation over anon-optimal path 302 such as FICON.

Referring to FIGS. 4 and 5, different conditions may exist on a storagesystem 110, such as the IBM DS8000™ enterprise storage system 110, thatmay make executing read requests and write requests more efficient ondifferent communication paths 300, 302. Functionality may be provided todetect such conditions and communicate this information to a host system106 so that the host system 106 can use the most efficient communicatepath 300, 302 to complete an I/O operation. For example, it generallydoes not make sense to use a higher performance communication path 300only to have an I/O operation fail or timeout and then have to retry theI/O operation using the lower performance communication path 302. Insuch cases, it would have been better and more efficient to use thelower performance communication path 302 in the first place.

Systems and methods in accordance with the invention may monitor variousconditions on the storage system 110. These conditions may, in manycases, indicate the availability or utilization of resources on thestorage system 110. Certain conditions may indicate that it would bemore efficient to execute read requests over a certain communicationpath 300, 302, while other conditions may indicate that it would be moreefficient to execute write requests over the same communication path300, 302. If a first set of conditions are satisfied (indicating certainresources are available, unavailable, scarce, plentiful, etc.),functionality in accordance with the invention may increase a ratio ofread requests to write requests that are transmitted over a higherperformance communication path 300 (e.g., a zHyperLink communicationpath 300) and reduce a ratio of read requests to write requests that aretransmitted over a lower performance communication path 302 (e.g., azHPF communication path 302), as shown in FIG. 4. Similarly, if a secondset of conditions are satisfied (indicating certain resources areavailable, unavailable, scarce, plentiful, etc.), functionality inaccordance with the invention may decrease a ratio of read requests towrite requests that are transmitted over a higher performancecommunication path 300 (e.g., a zHyperLink communication path 300) andincrease a ratio of read requests to write requests that are transmittedover a lower performance communication path 302 (e.g., a zHPFcommunication path 302), as shown in FIG. 5.

Referring to FIG. 6, in certain embodiments, a ratio adjustment module600 may be provided to adjust the ratio of read requests and writerequests that are transmitted over the communication paths 300, 302. Asshown, the ratio adjustment module 600 may include various sub-modulesto provide various features and functions. The ratio adjustment module600 and associated sub-modules may be implemented in hardware, software,firmware, or combinations thereof. The sub-modules are presented by wayof example and not limitation. More or fewer sub-modules may be providedin different embodiments. For example, the functionality of somesub-modules may be combined into a single or smaller number ofsub-modules, or the functionality of a single sub-module may bedistributed across several sub-modules.

As shown, the ratio adjustment module 600 includes one or more of aconditions module 602, threshold module 604, determination module 606,and adjustment module 608. The conditions module 602 may be configuredto establish one or more conditions that may be monitored and used todetermine whether to increase or reduce utilization of a particular I/Oprocessing technique. The conditions may include, for example, readcache hit ratio, read response times, write response times, queuing ofsegments in non-volatile storage of the storage controller 200, anamount or percentage of read request rejections, an amount or percentageof write request rejections, and the like. In general, the conditionsmay be indicators of resource utilization and availability within thestorage system 110. The availability or scarcity of certain resourcesmay favor execution of read requests over certain communication paths300, 302, while the availability or scarcity of other resources mayfavor execution of write requests over certain communication paths 300,302.

The threshold module 604 may establish thresholds for the conditionsestablished by the conditions module 602. In general, the thresholds mayindicate when a condition (e.g., cache hit ratio, response time,rejection rate, etc.) has reached a limit, which may in turn affect theperformance of read requests and/or write requests. In such cases, itmay be advantageous to adjust the ratio of read requests and writerequests that are transmitted over certain communication paths 300, 302to optimize the system. The thresholds may be expressed in terms of anumber, percentage, comparison relative to another metric, or the like.

Once the conditions and associated thresholds are established, thedetermination module 606 may monitor the storage system 110 anddetermine which thresholds have and have not been reached, and thus,which conditions have and have not been satisfied. The adjustment module608 may make adjustments based on which conditions have been satisfied.For example, if a first set of conditions are satisfied, the adjustmentmodule 608 may increase a ratio of reads relative to writes over ahigher performance communication path 300 and decrease a ratio of readsrelative to writes over a lower performance communication path 302.Similarly, if a second set of conditions are satisfied, the adjustmentmodule 608 may decrease a ratio of reads relative to writes over ahigher performance communication path 300 and increase a ratio of readsrelative to writes over a lower performance communication path 302.

FIG. 7 is a process flow diagram showing one embodiment of a method 700for increasing a ratio of reads relative to writes over a higherperformance communication path 300, such as a zHyperLink communicationpath 300. Such a method 700 may be executed by a storage system 110 suchas that illustrated in FIG. 2. As shown, the method 700 tests 702, 704,706, 708, 710, 712, for a plurality of conditions. In the illustratedembodiment, the method 700 determines 702 whether a read cache hit ratiois above a selected threshold (e.g., 80 percent); determines 704 whetherread response times are below a selected threshold (e.g., 30microseconds); determines 706 whether write response times are above aselected threshold (e.g., 50 microseconds); determines 708 whethernon-volatile storage (NVS) in the storage system 110 is queuing NVSsegments (i.e., the NVS, which caches write data, is currently full andcannot receive additional write data without freeing up space);determines 710 whether the percentage of reads that are rejected isbelow a selected threshold (e.g., 20 percent); and determines 712whether the percentage of writes that are rejected is above a selectedthreshold (e.g., 30 percent).

In the illustrated embodiment, if all of the conditions are satisfied,the method 700 sends 714 a hint to the host system 106 to, whentransferring data over the higher performance communication path 300,prefer read requests over write requests and/or increase the ratio ofread requests relative to write requests. In other embodiments,satisfaction of some subset of the illustrated conditions, orsatisfaction of additional or different conditions, may cause the method700 to send 714 a hint to prefer read requests over write requestsand/or increase the ratio of read requests relative to write requests.

FIG. 8 is a process flow diagram showing one embodiment of a method 800for decreasing a ratio of reads relative to writes over a higherperformance communication path 300, such as a zHyperLink communicationpath 300. Such a method 800 may be executed by a storage system 110 suchas that illustrated in FIG. 2. As shown, the method 800 tests 802, 804,806, 808, 810, 812, for a plurality of conditions. In the illustratedembodiment, the method 800 determines 802 whether a read cache hit ratiois below a selected threshold (e.g., 70 percent); determines 804 whetherread response times are above a selected threshold (e.g., 30microseconds); determines 806 whether write response times are below aselected threshold (e.g., 50 microseconds); determines 808 whethernon-volatile storage (NVS) in the storage system 110 is not queuing NVSsegments (i.e., the NVS, which caches write data, is not full and canreceive additional write data); determines 810 whether the percentage ofreads that are rejected is above a selected threshold (e.g., 30percent); and determines 812 whether the percentage of writes that arerejected is below a selected threshold (e.g., 20 percent).

In the illustrated embodiment, if all of the conditions are satisfied,the method 800 sends 814 a hint to the host system 106 to, whentransferring data over the higher performance communication path 300,prefer write requests over read requests and/or decrease the ratio ofread requests relative to write requests. In other embodiments,satisfaction of some subset of the illustrated conditions, orsatisfaction of additional or different conditions, may cause the method800 to send 814 a hint to prefer write requests over read requestsand/or decrease the ratio of read requests relative to write requests.

The flowcharts and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowcharts or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the Figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. Other implementationsmay not require all of the disclosed steps to achieve the desiredfunctionality. It will also be noted that each block of the blockdiagrams and/or flowchart illustrations, and combinations of blocks inthe block diagrams and/or flowchart illustrations, may be implemented byspecial purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

The invention claimed is:
 1. A method for dynamically balancing I/Oworkload, the method comprising: transmitting, from a host system to astorage system, read requests and write requests over a higherperformance communication path and a lower performance communicationpath; determining whether a first set of conditions and a second set ofconditions are satisfied on at least one of the host system and thestorage system; upon determining that the first set of conditions issatisfied, increasing a first ratio of read requests to write requeststhat are transmitted over the higher performance communication path anddecreasing a second ratio of read requests to write requests that aretransmitted over the lower performance communication path; and upondetermining that the second set of conditions is satisfied, decreasingthe first ratio of read requests to write requests that are transmittedover the higher performance communication path and increasing the secondratio of read requests to write requests that are transmitted over thelower performance communication path.
 2. The method of claim 1, whereinthe higher performance communication path is a zHyperLink communicationpath.
 3. The method of claim 1, further comprising providing, from thestorage system to the host system, a hint indicating whether to increaseor decrease at least one of the first ratio and the second ratio.
 4. Themethod of claim 1, further comprising providing, from the storage systemto the host system, information indicating whether the first and secondsets of conditions are satisfied.
 5. The method of claim 1, whereindetermining whether the first and second sets of conditions aresatisfied comprises determining whether a read cache hit ratio is aboveor below a selected threshold.
 6. The method of claim 1, whereindetermining whether the first and second sets of conditions aresatisfied comprises determining whether at least one of a read responsetime and a write response time is above or below a selected threshold.7. The method of claim 1, wherein determining whether the first andsecond sets of conditions are satisfied comprises determining whether atleast one of read rejections and write rejections are above or below aselected threshold.
 8. A computer program product for dynamicallybalancing I/O workload, the computer program product comprising anon-transitory computer-readable storage medium having computer-usableprogram code embodied therein, the computer-usable program codeconfigured to perform the following when executed by at least oneprocessor: transmit, from a host system to a storage system, readrequests and write requests over a higher performance communication pathand a lower performance communication path; determine whether a firstset of conditions and a second set of conditions are satisfied on atleast one of the host system and the storage system; upon determiningthat the first set of conditions is satisfied, increase a first ratio ofread requests to write requests that are transmitted over the higherperformance communication path and decrease a second ratio of readrequests to write requests that are transmitted over the lowerperformance communication path; and upon determining that the second setof conditions is satisfied, decrease the first ratio of read requests towrite requests that are transmitted over the higher performancecommunication path and increase the second ratio of read requests towrite requests that are transmitted over the lower performancecommunication path.
 9. The computer program product of claim 8, whereinthe higher performance communication path is a zHyperLink communicationpath.
 10. The computer program product of claim 8, wherein thecomputer-usable program code is further configured to provide, from thestorage system to the host system, a hint indicating whether to increaseor decrease at least one of the first ratio and the second ratio. 11.The computer program product of claim 8, wherein the computer-usableprogram code is further configured to provide, from the storage systemto the host system, information indicating whether the first and secondsets of conditions are satisfied.
 12. The computer program product ofclaim 8, wherein determining whether the first and second sets ofconditions are satisfied comprises determining whether a read cache hitratio is above or below a selected threshold.
 13. The computer programproduct of claim 8, wherein determining whether the first and secondsets of conditions are satisfied comprises determining whether at leastone of a read response time and a write response time is above or belowa selected threshold.
 14. The computer program product of claim 8,wherein determining whether the first and second sets of conditions aresatisfied comprises determining whether at least one of read rejectionsand write rejections are above or below a selected threshold.
 15. Asystem for dynamically balancing I/O workload, the system comprising: atleast one processor; at least one memory device operably coupled to theat least one processor and storing instructions for execution on the atleast one processor, the instructions causing the at least one processorto: transmit, from a host system to a storage system, read requests andwrite requests over a higher performance communication path and a lowerperformance communication path; determine whether a first set ofconditions and a second set of conditions are satisfied on at least oneof the host system and the storage system; upon determining that thefirst set of conditions is satisfied, increase a ratio of read requeststo write requests that are transmitted over the higher performancecommunication path and decrease a second ratio of read requests to writerequests that are transmitted over the lower performance communicationpath; and upon determining that the second set of conditions issatisfied, decrease the ratio of read requests to write requests thatare transmitted over the higher performance communication path andincrease the second ratio of read requests to write requests that aretransmitted over the lower performance communication path.
 16. Thesystem of claim 15, wherein the higher performance communication path isa zHyperLink communication path.
 17. The system of claim 15, wherein theinstructions further cause the at least one processor to provide, fromthe storage system to the host system, a hint indicating whether toincrease or decrease at least one of the first ratio and the secondratio.
 18. The system of claim 15, wherein the instructions furthercause the at least one processor to provide, from the storage system tothe host system, information indicating whether the first and secondsets of conditions are satisfied.
 19. The system of claim 15, whereindetermining whether the first and second sets of conditions aresatisfied comprises determining whether a read cache hit ratio is aboveor below a selected threshold.
 20. The system of claim 15, whereindetermining whether the first and second sets of conditions aresatisfied comprises determining whether at least one of a read responsetime and a write response time is above or below a selected threshold.